This invention pertains generally to a method for producing high pressures, by converting electric potential to hydrodynamic force, that requires no moving mechanical parts and particularly to the use of electro-osmotic flow to produce a high pressure system for compressing and manipulating fluids, in general, and in packed microchannels and capillaries, in particular.
The phenomenon of electro-osmosis, in which the application of an electric potential to an electrolyte in contact with a dielectric surface produces a net force on a fluid and thus a net flow of fluid, has been known since Reuss in 1809. The physics and mathematics defining it and its associated phenomenon streaming potential, both part of a larger class of electrochemical phenomena, namely electrokinetic effects, have been extensively explored, Introduction to Electrochemistry, S. Glasstone, 1942, pp. 521-529 and R. P. Rastogi, "Irreversible Thermodynamics of Electro-osmotic Flow", J. Sci. and Industrial Res., 28, 284, 1969. In like manner, electrophoresis, the movement of charged particles through a stationary medium under the influence of an electric field, has been extensively studied and employed in the separation and purification arts.
The use of electro-osmotic flow has been wide spread and has found wide ranging applications in chemical analysis. The use of electro-osmostic flow for fluid transport in packed bed capillary chromatography was first documented by Pretorius, et. al., "Electro-osmosis--A New Concept for High-Speed Liquid Chromatography", J. Chromatography, 99, 23-30, 1974. Although the possibility of using this phenomenon was recognized two decades ago, the effective use of this method to perform chemical analysis has only recently been demonstrated and has just begun to provide commercial utility.
Except for very general references to the fact that pressures generated by electro-osmotic flow were linearly proportional to the applied voltage (cf. Dasgupta and Liu, Analytical Chemistry, 1194, 66, 1793 and Theeuwes U.S. Pat. No. 3,923,426 at col. 1 line 23) there appeared to be no recognition in the prior art that electro-osmosis could be used to generate high pressure. Experimental studies that did explore the relationship between electroosmosis and pressure, generally studies of streaming potential, were limited to pressures below 1 psi (Rastogi, ibid. and Cooke, J. Chem. Phys., 1995, 23, 2302). Moreover, Rastogi, ibid., 291, has shown that the then recognized linear dependence of electro-osmotic pressure and applied electric potential begins to fail at voltages of about 300 to 400 Volts and pressures above about 0.2 to 0.3 psi and, in fact, pressure begins to approach an asymptote of between 0.3 to 0.4 psi at an applied electric potential on the order of 600 Volts. Thus, prior art did not recognize and, in fact, taught away from being able to achieve pressures above about 1 psi by means of electro-osmosis. It is believed that the cause of the non-equilibrium pressure/applied electric potential effects observed in earlier work may be the result of using capillaries having too large a diameter and/or solutions having too high a conductivity which can cause undesirable heating of the electrolyte to the point where boiling and bubble formation can take place.
High-performance liquid chromatography (HPLC) is an established analytical technique that relies on high-pressure mechanical pumps (generally a gear- or cam-driven pump capable of generating pressures in excess of 5,000 psi) to drive a fluid sample through a specially prepared column. The HPLC separation medium or stationary phase is typically a thick bed packed with fine particles. Fused silica beads are often used as an HPLC column packing material. However, fused silica alone is not in general a good separation medium. Usually, a special coating is applied to the particles or the particles themselves are porous. HPLC columns can also be packed with special polymers or resins. Regardless of the column packing used, the HPLC column presents a very large resistance to flow, hence the need for high pressures to drive the sample being analyzed through the system. In HPLC the high pressure pump itself can not only comprise a substantial component of the system but also, because of the need to fill the pump as well as attendant reservoirs and plumbing, can require the use of relatively large samples. However, this proves to be particularly cumbersome in those instances where it is desired to run a second sample or samples in parallel in columns having different stationary phases because of the requirement for separate pumps for each column. Finally, there is a desire to have field-portable instruments which has been frustrated by the size, bulk, and power consumption of conventional HPLC pumps as well as the large amounts of sample required.
Conventional HPLC systems typically employ separation columns of about 3-5 mm in diameter and flow rates .apprxeq.3-5 mL/min. However, miniaturization of the separation column (microbore columns) offers several advantages, including improved efficiency, mass detection sensitivity, low solvent consumption, small sample quantity, and easier coupling to detector such as mass spectrometers and flame-based detectors and several analytical methods using miniaturized or capillary columns have been developed for micro-HPLC and capillary electrochromatography (CEC). These columns generally have inside diameters of 1 mm or less.
Commercially available HPLC systems typically employ piston or cam-driven pumps to pressurize the column. However, it is difficult to adapt these pumps to provide the low flow rates under high pressure required for microbore HPLC systems. As a practical matter, cam-driven pumps with the desired stroke ratios cannot be designed for flow rates lower than about 50 .mu.l/min. A further disadvantage of cam-driven pumps is that a single pump can only provide a limited range of flow rates. This is because different flow rate ranges require cams of substantially different size and the position of the cam relative to the motor and piston is determined by the cam dimensions. Changing the positions of the motor and piston to accommodate a cam of different size is impractical because of the sensitive alignment required in piston pumps.
A alternate approach for pumping in microbore HPLC systems is the syringe-type piston pump. While this type of pump is capable of delivering solvent at a few .mu.l/min flow rate it is difficult to maintain a constant flowrate due to the continuously changing flow resistance.
What is required is a system that will provide pressure driven flows at constant and controllable flow rates, wherein the flow rate can be in the range of mL/min to .mu.l/min. Further, the system must be compatible with microbore columns and the desire for small sample quantity, low solvent consumption, improved efficiency, the ability to run samples in parallel, and field portability.